Ripening Of Particles Research Articles

Implementation of Precipitation and Ripening of Second-Phase Particles in the Constitutive Modelling of Creep

Recent developments in constitutive modeling make it possible to directly incorporate microstructural features of a material in the constitutive equations describing its mechanical behavior. For a precipitation hardened material, the most relevant microstructural characteristic is the average particle spacing in the dislocation glide plane. A model describing the effect of this parameter on the strain hardening and creep behavior proved to have a good predictive capability for the case when the particle arrangement does not change during the deformation. This becomes particularly important for high-temperature deformation when an interplay between second-phase precipitation and particle ripening may produce a complex time dependence of the particle strengthening effect. A reliable description of the deformation behavior then requires the use of a constitutive model in which the kinetics of the particle evolution is accounted for. Such an attempt can be based on the model developed by Mecking and Estrin, in which the dislocation density is treated as a single internal variable. The formulation contains the distance of free dislocation glide, which is affected by the presence of second-phase particles. The new feature is that a variable second-phase particle spacing is now considered. The two concurrent processes, viz. precipitation and ripening of particles, aremore » described in a unified way in that the time dependence of the average particle spacing is presented in an analytical form. Schwarze et al. tried to apply this concept modeling the mechanical behavior of INCOLOY alloy 800H, a carbide precipitation strengthened high temperature alloy. For the present paper the model was developed more rigorously and simplifying assumptions, which may be allowed at lower temperatures, are avoided.« less

The study of various styrene-butadiene copolymers as compatibilisers in bulk blends of polystyrene/polybutadiene

Bulk blends, produced by compositional quenching, were used to examine the effectiveness of different styrene-butadiene copolymers as interfacial agents in blends of polystyrene (PS) with polybutadiene (PB). Each polymer blend was compression molded for various times to allow particle ripening and was then tested for impact strength. The most effective compatibiliser proved to be a long, asymmetric diblock, which could entangle in both homopolymer phases. Diblocks with short PS segments were inadequate as interfacial agents, presumably because their short PS blocks were easily pulled from the matrix. Random copolymers with a composition near 50/50 were successful as interfacial agents, but the impact strength declined with a decrease in PS composition of the copolymer. A commercial multiblock copolymer, Phillips K-Resin®, also proved to be reasonably effective compatibiliser, giving better impact strength than the HIPS control.

A model for calculation of isothermal kinetics of the nucleation-and-growth type phase separation in the course of one-step heat treatment

A model is described for calculating the kinetics of a diffusion-limited nucleation-and-growth type phase separation. A homogeneous melt of glass is cooled to a temperature, at which the nucleation, growth and Ostwald ripening of particles of a new phase are possible. In the model, a spatial inhomogeneity of nucleation rate, caused by the formation of diffusion zones around particles, and an effect of interaction of these zones on growth of particles are taken into account. Criteria for application of the model are formulated and parameters of the model are calculated for a model binary glass. A conclusion is drawn that the proposed model is applicable to systems where supersaturation is not too high. In particular, nucleation must not be so plentiful as to suppress a growth from supersaturated solution and to initiate Ostwald ripening at the nucleation stage. It is noted that such situations are observed in experiments on phase separation in glasses. The model seems to be applicable to some other types of phase transformations as well.

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Life tests of polymer‐electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects the majority of the platinum to conditions that sustain particle growth. X‐ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m2/g Pt eventually stabilize to about 40 to 50 m2/g in the cathode and 60 to 70 m2/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributable to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

Kinetics OF the ammonia synthesis over Fe/TiO 2, hydrazine-pretreated Fe/TiO 2, and hydrazine-pretreated alkali-promoted Fe/TiO 2 catalysts

The kinetics of the synthesis of ammonia from its elements over Fe/TiO 2, an hydrazine-pretreated Fe/TiO 2, and hydrazine-pretreated alkali-promoted Fe/TiO 2 catalysts was studied in a flow microreactor at 101 kPa pressure. A significant improvement in the accuracy and precision of kinetic parameters extracted from the data was achieved by modifying the kinetic model to account for deactivation of supported catalyst particles by Ostwald ripening. The modified form of the kinetic model also yielded information about the particle ripening kinetics. Pretreatment of Fe/TiO 2, with hydrazine increased the ammonia synthesis turnover frequency by more than an order of magnitude over unpretreated Fe/TiO 2. The ammonia partial pressure dependence and apparent activation energy over hydrazine-pretreated Fe/TiO 2 were more representative of iron uninfluenced by the strong metal-support interaction (SMSI) which occurs in metal-titanic systems. In situ CO chemisorption measurements following the ammonia synthesis kinetics measurements showed higher CO uptake with hydrazine-pretreated Fe/TiO 2 than with unpretreated Fe/TiO 2. The increased turnover frequency, altered kinetic parameters, and higher CO uptake suggest that hydrazine pretreatment inhibited the onset of SMSI, which is attributed to titanium nitride formation on the support surface. Addition of the alkali promoters K and Cs to the catalysts not only increased the turnover frequency and decreased the apparent activation energy and ammonia partial pressure dependence, but acted to stabilize supported iron particles against growth by Ostwald ripening. The data suggest that physical covering of the surface by alkali inhibits Ostwald ripening of iron particles by blocking dissociation of iron atoms from supported particles thus suppressing their migration over the support surface to form larger particles.

On the quasiperiodical (modulated) structure in FeBe alloys

The precipitation phenomenon in supersaturated solid solutions of Fe-Be alloys (16.5 and 19.5 at. percent Be) was investigated during aging at 350 to 500/sup 0/C. It was found that the quasiperiodical distribution on the nodes of the simple cubic lattice observed in the investigation is formed not at the early stages of aging, but at the stage of Oswald ripening of particles. The total results of the statistical treatment of the measurements are plotted in the form of the curves of the density of the particle distribution with respect to one another. These curves represent the positions of the sections of the maxima at the half-height for the first and the second neighbors (in the plane (001)). The analysis of the experimental data permits the conclusion that the observed distribution of the particles is due to the interaction between elastic distortion fields around these particles and is connected with the sharp minimum of the elastic energy for definite crystallographic directions, namely for the (100) directions.